The present invention is directed to an apparatus and a method for guiding a fibrous web from a press section to a drying section, such as guiding a paper web from the press section of a paper machine to the drying section thereof. More particularly, the present invention is directed to such an apparatus and method for guiding a fibrous web from a press section to a drying section, in which the web is guided and supported on a drying wire as the web is guided into the drying section, such as around an initial group of drying cylinders.
After pressing, a fibrous web such as a paper web prepared in a paper machine, is extremely weak. A large number of breaks in the web tend to occur in the run thereof between the press section and a first drying cylinder within a drying section. Other dubious points in this regard where such breaks in a paper web tend to occur, include, e.g., spaces between individual drying sections or between individual groups of drying cylinders, or spaces occuring between successive sections in a paper-making machine. In other words, a web is prone to breaking or tearing at all locations where the web itself runs unsupported, i.e. in a free or open draw.
Moreover, since the last roll in a press section is usually a stone roll, it is difficult to detach a running web from this roll because the web will tend to follow the very smooth face of the stone roll.
The known "SYM-PRESS" press section, which can be applied in accordance with the apparatus and method of the present invention, comprises a closed roll combination having four press rolls, in which the first nip is formed between a hollow-faced roll and a suction roll, and is provided with two pressing felts. The second single-felt nip of the "SYM-PRESS" press section is formed between the suction roll and a center roll of the press section. The center roll of the "SYM-PRESS" press section is a smooth-faced stone roll, the third, final nip of the press section being formed in connection with this stone roll and being provided with its own felt. The running fibrous web is detached from the smooth face of the stone roll in the "SYM-PRESS" press section by extending or stretching the web in the running direction.
The running web had to be extended extensively in the prior art, by up to two to three percent in the running direction thereof, in order to detach the web from the stone roll. Such extensive stretching or extending was necessary so that it would be possible to control fluttering of the running web that would occur between the press section and the drying section. However, a paper-making machine should be run with minimum differences in speeds of the running webs, so that the web itself should not lose any of its strength. Additionally, the great degree of extending resulted in many later disadvantages and difficulties, such as in connection with coating of the fibrous web or paper so prepared.
A prior art publication by Scapa-Porrit Ltd., entitled "Felts and Fabrics. Sheet Flutter in the Dryer Part", suggests a method for detaching the web from the stone roll by means of a suction roll, in which the web is passed and supported all the way from the press section onto a first drying cylinder in a drying section. However, the running web is detrimentally extended in the method suggested in this publication.
Moreover, this particular prior art method disclosed in the Scapa-Porrit Ltd. publication, has proven much more difficult to apply in practice, then previously expected. Suction alone is not sufficient for detaching a fibrous web from a stone roll. Moreover, it has been found that the web will not remain in contact with the drying wire without additional support, and a closing nip formed by a first drying cylinder in the drying section with the drying wire, causes wrinkles in the web, in particular along the sides of the running web. If a felt is used instead of a drying wire, then problems such as rewetting of the running fibrous web as well as contamination of the suction rolls, have been encountered.
Attempts have recently been made in the case of paper machines, to reduce the overall number of suction rolls, and even aim at entirely eliminating the need for any suction rolls, since such suction rolls are costly, require a tremendous amount of energy and maintenance, and easily malfunction or break down. Threading of the end of the fibrous web is also difficult when a suction roll is used for detaching the web from a stone roll, since during the threading of the web, the suction roll also inspires ambient air from surrounding areas not covered by the web, so that no suctioning effect is generated on the tail end of the web.